Paul,
you iwll see this error is pretty frequent on the internet,
in short the issue is when we run squid -k rotate to Parse configuration file, then send signal to running copy and exit.
When we do that, sometime the running process is not anymore the one pointed by in the pid file : /var/log/squid/squid.pid
looking at the squid log could help determining why this is happening :
- race condition
- squid has crashed
a possible issue I see is after log rotation of /var/log/squid/squid.log, no new /var/log/squid/squid.log seems created on one of my test server